Multicoaxial line cables



Aug. 22, 19 1 M. E. HINES ET AL MULTICOAXIAL LINE CABLES 2 Sheets-Sheet1 Filed OCT,- 8, 1959 lNl/EN TO/QS K A TTORNFV Aug. 22, 1961 M. E. HINESET AL 2,997,519

MULTICOAXIAL LINE CABLES Filed Oct. 8, 1959 2 Sheets-Sheet 2 M. E. H/NESWVEMOAS a. RA/SBECK wwh ATTORNEV United States Patent 2,997,519MULTICOAXIAL LINE CABLES Marion E. Hines, Summit, and Gordon Raisbeck,Bernards Township, Somerset County, N.J., assignors to Bell TelephoneLaboratories, Incorporated, New York, N.Y., a corporation "of New YorkFiled Oct. 8, 1959, Ser. No. 845,178 9 Claims. (Cl. 174-28) Thisinvention deals with electric cables for highfrequency transmission andmore particularly with cables comprising a plurality of coaxial lines.

Foreseeable abrupt increases in the employment of highfrequency pulsesystems, for data transmission and the like, have focused attention onthe need for cables comprising coaxial lines that may be produced bothelliciently and economically. With known methods and structure the costof producing coaxial cables is a factor which in many instances tends tolimit their use. Each individual coaxial linemust be fabricated as aseparate operation. Additionally, the operation of combining theindividual lines into a single cable, which is both costly and timeconsuming, subjects the lines to stresses which tend to reduce theuniformity of both their electrical and mechanical characteristics. Afurther disadvantage of prior art structures is that the employment ofcylindrical outer conductors results in the formation of open spacesbetween such conductors when they are packed together in cable form.Pressure on the outside of the cable tends to force the individualcyclindrical members into the open spaces which is conducive tomechanical deformation and attendant variations in transmissioncharacteristics.

A general object of the invention, therefore, is to provide an improvedmulticoaxial line cable that can be produced both economically andefliciently.

A more specific object lof the invention is to provide a multicoaxialline cable that positively limits relative movement between individuallines in the cable while still retaining a high degree of flexibility inthe cable as a whole.

A further object of the invention is to provide means by which thecenter conductors of a plurality of coaxial lines comprising a cable maybe positioned within their respective outer conductors by a singleoperation.

An additional object of the invention is to provide a multiccaxial linecable which is characterized by an optimum combination of both physicaland electrical properties.-

The principles of the invention are based in part on the realizationthat the configuration of the outer conductor of a coaxial line may bechanged substantially from its conventional cylindrical form withlittle, if any, impairment of its electrical properties and with amarked improvement in its mechanical properties. Specifically, theprinciples of the invention contemplate the employment of a hexagonalform for the outer conductor of each of the coaxial lines in amulticoaxial line cable. With the employment of outer conductors whichare hexagonal in cross section, it is apparent that a number of coaxiallines may be closely and securely packed together after the fashion ofthe classical honeycomb configuration.

The invention uniquely turns to account the employment of closely packedhexagonal outer conductors in still another way. In the construction ofa cable, in acoordance with the principles of the invention, the outerconductors are formed by a plurality of conductive strips each of whichis equal in length to the desired length of the cable or cable section.Each of the strips is bent along its length to form a plurality ofparallel, adjacent, longitudinal, three-sided, half-hexagonal channelsor grooves which are successively open on opposite sides of the strip,Two of such strips pla ed in the proper Hilliadit-inter! Aug. 22, 1961posed relation form a single row of coaxial cable outer conductors, eachhaxagonal in cross section. By stacking additional strips together withthe two strips described, the number of outer conductors in the Cablemay be increased as desired and the over-all cross section of the cableso formed is a plurality of closely packed hexagons or a so-calledhoneycomb array. It is evident then that according to the principles ofthe invention, a plurality of coaxial line outer conductors may befabricated and assembled in a single step, that is by stacking togetherappropriately bent strips, in contrast to the multiple operationsrequired heretofore. Moreover, since each of the heaxgonal outerconductors is mechanically discontinuous, although electricallycontinuous, ready access is provided for the positioning of innerconductors, either by conventional means or in accordance with aparticular aspect of the invention.

In one illustrative embodiment of the invention a cable comprising abundle of hexagonal coaxial lines with outor conductors assembled asdescribed is bound together by a covering of insulation. Since bothedges of each of the outer conductor strips abut the insulatingcovering, lateral movement between individual coaxial lines isprohibited.

Various arrangements are employed in the prior art to increase theflexibility of coaxial cable. For example, some cables employ outerconductors comprising helically wound copper strips and others employouter conductors which are corrugated. One aspect of the instantinvention is the employment of corrugations on the outer conductors ofeach of a plurality of coaxial lines in a cable with the corrugations soformed that they impart a good degree of flexibility to the cable and atthe same time interlock with adjacent outer conductors so as to prohibitlongitudinal movement between adjacent conductors, or more preciselybetween adjacent outer conductor strips.

In an array of outer conductors which conform in structure to theprinciples of the invention, the inner conductors may be positioned byconventional means, as suggested above. For example, each individualinner conductor may be threaded through its respective outer conductorand retained in spaced relation thereto by means of insulating beads orwashers. In accordance with a further principle of the invention,however, all of the inner conductors corresponding to all of the outerconductors which are formed by two adjacent channeled strips may bepositioned in a single operation by turning to account the mechanical orstructural discontinuity of each of the outer conductors. Before the twoconductive strips which comprise a single row of outer conductors areplaced in juxtaposition, a relatively thin strip of suitable insulatingmaterial, which is comparable in length and width to the conductivestrips, is placed in juxtaposi tion with one of the conductive strips. Anumber of inner conductors are suitably atlixcd longitudinally on theinsulating strip, each being placed in such a position that it isconcentrically aligned with its respective outer conductor. Hence, thepositioning of a plurality of inner conductors may be effected readilyin a single operation.

One feature of the invention, therefore, is a honeycomb bundle ofcoaxial lines with outer conductors formed from a plurality of suitablybent or channeled strips of conductive material placed in juxtaposition.

A further feature of the invention is a means for positioning aplurality of inner conductors in a bundle of hexagonal coaxialconductors constructed as described, comprising an insulating strip towhich the inner conductors have been suitably aflixed.

A still further feature of the invention is the employment ofinterlocking corrugated surfaces on the juxtaposed sides of all thehexagonal outer conductors which form a honeycomb bundle of coaxiallines.

The invention, together with additional objects and features thereof,will be fully apprehended from a consideration of the following detaileddescription and accompanying drawings of particular embodiments, inwhich:

FIG. 1 is a sketch of two longitudinally bent strips designed, inaccordance with the invention, to formthe outer conductors of aplurality of coaxial lines;

FIG. 2 is a sketch of the strips of FIG. 1 placed in juxtaposition withan inner conductor conventionally positioned in each outer conductor;

FIG. 3 is a cross section view, in perspective, of a cable comprising aplurality of hexagonal coaxial lines;

FIG. 4 is a cross section view, in perspective, of a cable which employsinsulating strips to position the inner conductors of a plurality ofhexagonal coaxial lines;

FIG. 5 is a cross section view, in perspective, of a single hexagonalcoaxial line in which kinks of the inner conductor are employed toretain it in place;

FIG. 6A is a sketch of the outer conductor strips shown in FIG. 1 withcorrugations in each of the horizontal surfaces; FIG. 6B is a sketch ofthe outer conductor strips shown in FIG. 1 with corrugations in both thehorizontal and slant faces of the strips; and

FIG. 7 is a cross section view of a cable, in accordance with theinvention, with cylindrical conductors shown; for comparison purposes,inscribed in the several hexagonal outer conductors.

One of the more significant aspects of the invention, the use ofconducting strips to form hexagonal outer conductors, is illustrated inFIG. 1. The pair of electrically conducting strips 1 and 2 have eachbeen formed into a plurality of parallel, longitudinal, three-sidedgrooves or channels by a succession of 60 degree longitudinal bends.Each of the channels is formed by a group of four successive bends, andin each group the first and fourth bends, such as 3 and 4, are made in afirst direction, and

the second and third bends, such as 5 and 6, are made in a seconddirection opposite to the first. The material of the strips is normallymetallic and may advantageously be of copper, for example. The bendingoperations may be effected by any one of a number of means known in themetal working art, as by stamping, or by drawing the metal over an arrayof suitably formed rollers, for example.

The strips, formed in accordance with the invention as indicated above,may be viewed as a plurality of channels, such as 7, 8, 9, and 1%, equalin dimension, and equally spaced or, stated otherwise, the stripscomprise a series of adjacent channels of equal dimension with alternateones 7, 8, 9, and It} open on one side of the strip and intermediateones 11, 12, and 13 open on the opposite side of the strip. Theprinciples of the invention contemplate the employement of each of theoppositely facing adjacent channels as one half of the outer conductorof a coaxial line. This employment of the strips is shown in FIG. 2.

In FIG. 2 the two strips 1 and 2, also shown in FIG. 1, have been placedtogether so that each channel in each strip is in juxtaposition with anoppositely facing channel in the other strip so that a row of spacedouter conductors 14-, 15, 16, and 17 is formed, each hexagonal in crosssection. Four inner conductors 18, 19, 213, and 21 are also shown, eachplaced concentrically in a respective one of the outer conductors. Eachinner conductor 18, 19, 20, and 21 is fixed, conventionally, in thecenter of its respective outer conductor 14, 15, 16, and 17 by spacersor washers 22 through 29 which may be of insulating material such ashard rubber or polyethylene, for example. Alternatively, a soliddielectric such as polyethylene or a suitable foamed insulating materialmay be employed.

FIG. 3 shows an entire cable 30 comprising a total of 37 coaxial lineswhich in structure and in method of assembly conform to the principlesof the invention. A total of 14 electrically conductive strips have beenstacked together in the manner illustrated in FIGS. 1 and 2 to form thesubstantially cylindrical honeycomb bundle of coaxial lines The outerperiphery of a honeycomb bundle of coaxial lines embodying theprinciples of the invention may of course be adjusted to any desiredform by varying the number and relative positions of the individuallines. In FIG. 3 the proper concentric positioning of each of the centerconductors is attained by the use of a solid dielectric, such aspolyethylene, which in some instances may be more advantageous than theuse of washers, beads or spacers as shown in FIG. 2.

In any complete cable structure it is usual to employ an outer coveringor shield to promote both mechanical strength and electrical isolation.In FIG. 3, the outer covering 31, which may be of lead, for example, issubstantially cylindrical on its outer surface while its inner surfaceis of an irregular form'so that it fills in the'spaces around the outeredge of the honeycomb bundle. Alternatively, of course, the outersurface of the outer covering 31 may be made to conform to the outersurfaces of the honeycomb bundle.

FIG. 4 illustrates a particular feature of the invention which is a.unique means for positioning the inner conductors in a cable constructedin accordance with the principles of the invention. Three outerconductor strips 32, 33, and 34 are shown stacked together, in the samefashion illustrated by FIGS. 1 and 2, and relatively thin fiat strips ofinsulating material 35 and 36, such as polyethylene for example, havebeen placed between the con tacting horizontal surfaces of the adjacentbent strips 32 and 33, and 33 and 34, respectively. Along the length ofeach of the insulating strips the inner conductors, for example 37, 38,and 39, have been embedded or otherwise aflixed at the proper intervals.Stretching the insulating strings 35 and 36 before they are locked inplace by the pressure of the adjacent conducting strips 32, 33, and 34ensures the elimination of any sag in the insulating strips which inturn ensures that each of the inner conductors is accurately positionedconcentrically with relation to its respective outer conductor.

Any one of a number of means may be employed to ensure electricalcontinuity between the two halves of each outer conductor which wouldotherwise be insulatedly separated by the strips 35 and 36. For example,conductive staples 62 may be used, as shown, or, alternatively,conductive particles may be suitably embedded in the insulating strips35 and 36 to provide a conductive path betwen adjacent outer conductorstrips. Another method of ensuring electrical continuity between the twohalves of each outer conductor is the employment of insulating stripswith suitable gaps at points where contact between adjacent conductivestrips is desired. Still another method is the employment of transversestrips spaced sufi'iciently far apart to ensure'contact between adjacentconductive strips.

In accordance with the invention an additional means may be employed toensure accurate positioning of the inner conductors. Each of the bendsin the outer conductor strips may be made at an angle slightly greaterthan that required to produce the half-hexagonal channels shown in thedrawings. The resulting hexagonal outer conductors are thus slightlyelongated on the axes perpendicular to the insulating inner conductorstrips. In the process of assembly, pressure is applied to the strips,as indicated by the arrows in FIG. 4, and each hexagon is compressedslightly in a direction perpendicular to the insulating strips andexpanded slightly in a direction parallel to the insulating strips untileach interiorangle of each hexagonal outer conductor measures 60degrees; It is' apparent that such action serves to stretch eachinsulating strip transversely across the intenor of each outerconductor, thus giving somewhat greater accuracy to the concentricpositioning of each inner conductor in its respective outer conductor.

FIG. 5 illustrates an alternative method of supporting the innerconductor in a coaxial line constructed in accordance with theprinciples of the invention. As in FIG. 2, two electrically conductivestrips 1 and 2, each suitably bent to form a half-hexagonal channel,have been placed in juxtaposition to form a hexagonal outer conductor44. The center conductor 40 has been bent or kinked sharply at suitableintervals 41, 42, and 43, and each kink is wedged into a respective oneof the angles formed by adjacent sides of the outer conductor 44.Shifting the point of contact between the inner and outer conductor by120 degrees at each successive kink of the inner conductor ensuresuniform support for the inner conductor throughout the length of theline. The inner conductor 40 is, of course, insulated from the outerconductor 44 at each of the kinks 41, 42, and 43, which may be achievedby a small section of insulation at the appropriate points on the innerconductor as shown, or by insulating a small spot at each contact pointon the interior of the inner conductor. Changes in transmissionproperties resulting from the kinked inner conductor, as compared to astraight conductor, are relatively minor and can be readily compensatedfor.

While it is apparent that a honeycomb arrangement of closely packedhexagons, as shown for example in FIG. 3, precludes relative lateralmovement between adjacent conducting strips, since the edges of eachstrip are wedged against the outer covering 31, the arrangement does notafford similar resistance to relative longitudinal movement betweenadjacent conducting strips. Such relative movement is to be avoided,just as lateral movement, since repeated longitudinal shifting of theconducting strips with respect to one another may have an adverse effecton both the electrical and the mechanical properties of the cable. Inthe prior art, one method which is employed to avoid relative movementamong the individual cylindrical coaxial lines in a cable is to twistthe lines in spiral fashion. This method is objectionable, however,since the flexibility of the cable is decreased thereby and the linesare subjected to mechanical stresses by the twisting process which isconducive to the introduction of variations in the transmissioncharacteristics of the lines.

In accordance with the principles of the invention, this problem is metin the manner illustrated in FIG. 6A and FIG. 6B. FIG 6A shows a pair ofouter conductor strips 1 and 2 similar to the strips shown in FIG. 1. InFIG. 6A, however, each of the horizontal faces of each of the strips hasbeen impressed with corrugations so that when two corrugated surfacesare placed together they will interlock, thereby preventing relativelongitudinal displacement between adjacent conducting strips 1 and 2.The corrugations may of course take any one of 'a variety of forms, suchas sawtooth or sinusoidal for example, and still perform the functionfor which they are intended. It would appear that the corrugations inthe surfaces of the outer conductor would produce impedancediscontinuities. Such discontinuities may readily be avoided, however,by positioning successive indentations at intervals of less thanone-quarter Wavelength at the highest operating frequency on the line.Additionally, indentations on the top and bottom plane of an outerconductor may be offset with respect to each other which reduces stillfurther the possibility of creating impedance discontinuities.

The interlocking feature of the corrugations is effective even when theindentations are very shallow and thus changes in the transmissioncharacteristics of the line may be held to a minimum. Moreover, sincethe corrugations are uniform, changes in electrical properties broughtabout by the convex corrugations, such as increased capacitance, forexample, are eliminated, or at least substantially reduced, by thematching concave corrugations. In addition to the function of preventingrelative movement between adjacent conducting strips, corrugationsincrease the flexibility of each individual coaxial line and therebyincrease the flexibility of the cable as a whole. In applications wheremaximum cable flexibility is a desirable feature, corrugations may beimpressed on every face of the outer conductor strips, as shown in FIG.613, instead of restricting the corrugations to the horizontal surfacesas shown in FIG. 6A. When corrugations are impressed on all surfaces ofthe outer conductors, as shown in FIG. 6B, the indentations not onlyprovide predetermined points at which each coaxial line may flex, butalso they serve the function of providing additional conductor lengthfor a given length of transmission line so that the conductors on theoutside of a bend may stretch, thereby avoiding any sharp buckling.

It is of course recognized that the cylindrical configuration is in somerespects ideal for the outer conductor of a coaxial line since for aparticular attenuation per unit length, the actual volume enclosed by acylindrical conductor is less than for other configurations. While thisadvantage is clear for a single coaxial line, it tends to disappear whenthe physical size and transmission characteristics of a cable comprisinga plurality of coaxial lines with cylindrical outer conductors arecompared to those properties in a cable employing coaxial linesconstructed in accordance with the invention. Such a comparison is bestillustrated in FIG. 7. A total of seven closely packed hexagonal outerconductors 46 through 52 are shown encased in an outer covering 53. Forcomparison purposes each one of seven cylindrical conductors 54 through60 is inscribed in a respective one of the hexagonal conductors 46through 52. It may be observed that the total volume, or cross sectionalarea formed by enclosing the hexagonal conductors in the cylindricalouter covering 53, represents only a very insignificant increase overthe total volume or cross sectional area formed by enclosing thecylindrical conductors in the cylindrical outer covering 61. Moreover,in addition to the desirable mechanical features, as described above,which are afforded by a honeycomb bundle of coaxial lines, the hexagonalconductors 46 through 52 have preferred transmission characteristicsover the inscribed cylindrical conductors 54 through 60. Specifically,the capacitance of the hexagonal conductors is less since the averagedistance of the sides of each of the hexagonal conductors from itsrespective center conductor is of course greater than the distance ofeach cylindrical conductor from its respective inner conductor.Consequently, it may be observed that the features of the invention,when embodied in a cable comprising a plurality of coaxial lines,provide not only structural advantages but also advantages intransmission characteristics.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of this invention.Numerous other arrangements may be designed by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. A transmission cable including a plurality of coaxial lines eachcomprising an inner and an outer conductor, said outer conductorscomprising a plurality of electrically conductive strips each bentsuccessively to form a plurality of open, longitudinal, parallel,half-hexagonal channels, alternate ones of said channels being open onone side of said strip and intervening ones of said channels being openon the opposite side of said strip, means positioning said strips injuxtaposition thereby to form a closelypacked honeycomb bundle of saidouter conductors, each of said outer conductors being hexagonal in crosssection, and means positioning each of said inner conductors in arespective one of said outer conductors.

2. Apparatus in accordance with claim 1 wherein said inner conductorpositioning means comprises a plurality of strips of insulatingmaterial, each of said insulating ,7 strips being stretched between andsecured by a respective adjacent pair of said electrically conductivestrips, and means aifixing the inner conductors of each of said groupsof outer conductors to a respective one of said insulating strips.

3. A transmission cable including a plurality of groups of coaxiallines, each line comprising a cylindrical inner conductor and ahexagonal outer conductor, each of said groups of outer conductorscomprising a pair of juxtaposed electrically conductive strips eachformed into a plurality of successiv ly oppositely facing, open,halfhexagonal channels, means positioning each of said pairs ofconductive strips in juxtaposition thereby to form a closely-packedhoneycomb bundle of said hexagonal outer conductors, means interlockingeach of said conductive strips to the conductive strips adjacentthereto, and means positioning each of said inner conductors in arespective one of said outer conductors.

4. Apparatus in accordance with claim 3 wherein said interlocking meanscomprises corrugations on each of the surfaces on each of saidconductive strips which is in juxtaposed relation with a surface onconductive strips adjacent thereto.

5. A transmission cable including a plurality of groups of coaxiallines, each line comprising a cylindrical inner conductor and ahexagonal outer conductor, each of said groups of outer conductorscomprising a pair of juxtaposed electrically conductive strips eachformed into a plurality of successively oppositely facing, open,halfhexagonal channels, means positioning each of said pairs ofconductive strips in juxtaposition thereby to form a close1ypackedhoneycomb bundle of said hexagonal outer conductors, means ensuring apreselected degree of flexibility in said cable, said flexibilityensuring means in terlocking each of said conductive strips to theconductive strips adjacent thereto, and means positioning each of saidinner conductors in a respective one of said outer conductors.

6. Apparatus in accordance With claim 5 wherein said f5 flexibilityensuring means comprises corrugations on all surfaces of said conductivestrips.

7. Apparatus in accordance with claim 6 wherein said inner conductorpositioning means comprises a plurality of substantially flat,insulating strips each stretched be tween and secured by a respectiveadjacent pair of said conductive strips, and means afiixing the innerconductors of each of said groups to a respective one of said insulatingstrips.

8. Apparatus in accordance with claim 6 wherein said aifixing meanscomprises means for embedding said inner conductors in said insulatingstrips.

9. A plurality of outer conductors for a coaxial cable comprising aplurality of electrically conductive strips each including a pluralityof parallel, longitudinal, halfhexagonal channels, each of said stripsbeing placed in juxtaposition with at least one other of said stripswith complementary ones of said half-hexagonal channels incorrespondence, whereby each of said plurality of outer conductorsformed thereby is hexagonal in cross section, each comprising one ofsaid half-hexagonal channels from one of said strips and one of saidhalf-hexagonal channels from another of said strips.

References Cited in the file of this patent UNITED STATES PATENTS334,850 Denison Jan. 26, 1886 730,251 Gibbs June 9, 1903 1,132,676Murray et a1 Mar. 23, 1915 2,034,036 Green Mar. 17, 1936 2,223,116 MeyerNov. 26, 1940 2,614,172 Greenfield Oct. 14, 1952 2,636,923 Perzel Apr.28, 1953 2,796,589 Adams June 18, 1957 2,952,728 Yokose Sept. 13, 1960FOREIGN PATENTS 686,998 Germany Jan. 20, 1940 952,417 France May 2',1949

